An Overview of Nerve Injury Recovery
Associated with Muscles
Muscles have different ways to recover or adapt to nerve injuries. There are four types of nerve injuries that can occur. This section will discuss those injuries and how muscles adapt to the injury.
Functional Compression Nerve Block
The first nerve injury is called a functional compressive nerve block. This type of block we have all experienced when we rest our ulnar nerve (the “funny bone”) on a hard surface. This position allows the ulnar nerve to be compressed and the nerve then stops working. This is due to a lack of the normal chemical transmission through the nerve membrane. The outside of the hand “falls asleep”. When the pressure is removed, the chemical imbalance resolves and the nerve starts transmitting almost immediately. Sensation and muscle function then returns.
Functional Myelin Injury Nerve Block
The second injury is easily recoverable in function. This conduction block is due to an injury to the myelin sheath. The myelin sheath is the insulation found on larger nerves and is required to be intact to allow the signal to pass down these nerves. An injury to the myelin sheath with the nerve itself remaining intact will cause the conduction to be blocked but nerve cell itself remains normal. The injured myelin sheath will repair itself and the nerve will conduct normally again. Repair of this sheath requires between four to twelve weeks to fully recover.
Wallerian Degeneration (Loss of the Nerve Axon with an Intact Myelin Sheath)
In this type of motor nerve injury, the long body of the nerve (the axon) is injured but the myelin sheath (the insulation) remains intact. This will produce a situation called Wallerian Degeneration. In this setting, the nerve will regrow down its old pathway through this intact myelin sheath tunnel and reconnect to the muscle once again. This implies that the pathway for the nerve (the myelin sheath) is not damaged.
The motor nerve will grow down an intact myelin sheath at about one inch per month. As an example, an injury to the S1 nerve at the spinal level L5-S1 where a disc herniation typically occurs to injure the nerve means that this nerve needs to grow down the sheath more than two feet. That translates to 24 months at one inch a month.
The problem with this delay in reattachment is that the muscle without a nerve supply will become fibrotic (meaning useless) in a period of 12-18 months. Muscles that are eighteen inches or less from the nerve injury (assuming an intact myelin sheath) will still have a chance to function. Muscles in the thigh and the entire arm generally fit this description when the nerve is injured by spine compression (herniated disc or bone spur).
Unfortunately, the muscles in the lower leg and foot are generally too far from the injury to recover from a spinal issue in the lower back. This includes L5 and S1 nerves and some of the muscles supplied by the L4 nerves. There are some L5 and S1 innervated muscles (buttocks and thigh) that are close enough to recover in this type of nerve injury.
Nerve and Myelin Cell Death- Nerve Budding
The fourth type of motor nerve injury is found with more prolonged or severe compression that actually causes the nerve cell itself to die along with the myelin sheath. This nerve cannot regenerate and the muscle cell will remain without a nerve connection.
If any muscle cell has no nerve input for a period of time, the muscle generates a chemical signal for help. Any intact motor nerve in the immediate area will sprout and will send a new branch to this muscle cell. This budding process takes between ten to sixteen weeks. When the muscle cell is reconnected, this muscle cell can contract.
On the EMG, this appears as a more coarse contraction. Intact axons (nerve cells) can bud up to ten times. That is, a single nerve can pick up ten times the number of muscle cells it normally services.
If the nerve is so severely damaged, there may not be enough intact functioning nerve cells immediately around the surrounding muscle. These muscle cells will fibrose (die).
Nerve budding or sprouting also occurs in the situation where the nerve can still regenerate (see Wallerian Degeneration) but regeneration takes a longer period of time than the process of budding will take.
Another recovery function has to do not with muscle re-innervation but with muscle conditioning. The remaining muscle cells that continue to function can hypertrophy to compensate for the missing cells. I call this “Arnold Schwarzenegger compensation”. This is where physical therapy can help to condition and strengthen the remaining functioning muscle cells to compensate for the missing muscle cells.